If the Mosul Dam goes, a lake of water will submerge cities from Mosul to Baghdad and kill hundreds of thousands. This map shows the potential devastation.

The Iraqi government and an Italian engineering firm have finally reached a tentative agreement that could provide a long-term fix for the world’s most dangerous dam. But the tricky repairs needed to prevent a catastrophic failure at the Mosul Dam, in northern Iraq, could potentially make a bad situation even worse.

Mosul Dam, built in the early 1980s, has for decades been considered a ticking time bomb. Constructed on top of gypsum, limestone, and other minerals that dissolve when in contact with water, the dam has been plagued by the threat of collapse since even before it began operations. Six days a week for 30 years, engineers have pumped thousands of tons of grout under the dam to shore it up and prevent a catastrophic breach. U.S. Army Engineers famously called it “the most dangerous dam in the world.”

Late last week, Lt. Gen. Sean MacFarland, the top U.S. general in Iraq, warned that the dam is again in danger of collapsing, which would hurl a lake full of water down the Tigris River, flooding cities from Mosul to Baghdad and possibly killing hundreds of thousands of people.

U.S. military officers repeatedly sounded warnings about the dangers posed by the Mosul Dam after the 2003 U.S. invasion of Iraq. And worries about the dam’s integrity ratcheted up sharply in 2014, when the Islamic State briefly took control of the dam itself, after earlier having seized the nearby city of Mosul. Some experts fear that the Islamic State’s temporary takeover interrupted the protective work on the dam, increasing the risk it could break apart. Engineers and managers at the dam, in contrast, have said that the Mosul Dam faces no imminent threat.

There’s so much concern about the dam because the consequences of a breach would be horrific. Engineers have over the years modeled what would happen if the 11 billion cubic meters of water pent up behind the dam (that’s one-third as much as in Lake Mead, the largest U.S. reservoir, located in Arizona and Nevada) suddenly broke out downstream.

A team of researchers at Lulea University of Technology in Sweden recently compiled many of the simulations done for a dam breach, part of a wider study on all aspects of the ill-fated project. The researchers found that:

Within about four hours, Mosul would be facing a wave of water almost 80 feet high; flooding would cover about 28 square miles.

Within 22 hours, Saddam Hussein’s hometown of Tikrit would be hit with a 50-foot wall of water.

And within two days of a dam breach, Baghdad itself — 400 miles downriver — would have 13 or so feet of water all over the center of the city, and flooding would cover more than 80 square miles around the capital.

Scenarios like these have had big engineering firms angling to come up with a permanent solution; the nearly 100,000 tons of grout pumped under the dam so far are just a Band-aid. But there’s no consensus among experts on just what the best approach should be to finally fix the Mosul Dam.

On Tuesday, Italian Foreign Minister Paolo Gentiloni confirmed that the Iraqi government and the Trevi Group of Italy were close to finally signing a deal for long-term repair of the dam’s foundation. The Italian company has been trying to secure the Mosul contract since last year, but concerns about the Islamic State’s continued presence and questions about security at the work site have delayed any work so far. Gentiloni said on Tuesday that talks with Baghdad about the dispatch of up to 450 Italian troops to protect the site are well advanced.

The Trevi Group has carried out similar work at more than 150 other troubled dams, including in the United States. The Wolf Creek Dam in Kentucky, for example, was built on geology similar to that at the Mosul Dam and faced the same kind of seepage problem. Between 2006 and 2013, Trevi built a concrete wall under the dam’s embankment, known as a “cutoff wall,” that acts as a barrier to seepage and ends the threat of erosion caused when water hits limestone, gypsum, or other minerals.

“Putting a cutoff wall at Mosul is likely the only long-term solution to the problem,” John Rice, a professor of civil and environmental engineering at Utah State University and an expert on dam stability, told Foreign Policy. The big challenge at Mosul, he said, is that the dam will require a deeper cutoff wall than has ever been built in the world before, about 800 feet below the dam’s embankment.

“The construction of the cutoff will not be easy and, if done without proper precautions, could increase the probability of failure,” he said. But Iraq has few real alternatives: Grouting is a short-term fix at best and can be interrupted at any time due to the security situation on the ground.

Other experts have cautioned against trying to dig under the already unstable Mosul Dam. Nadhir Al-Ansari, a civil engineering professor at the Lulea University of Technology, who has spent years studying the dam, specifically recommended that the Iraqi government forget the idea of building a cutoff wall, warning in the study with his colleagues that it “is not only infeasible technologically and financially, but it could endanger the integrity of the dam itself.”

“I don’t think it is a good solution,” Al-Ansari told FP. He said a better way to prevent flooding from a catastrophic breach at Mosul Dam is to finally build a retaining dam downstream on the Tigris River, an expensive project that has been stalled for years.

A spokesman for the Trevi Group told FP that, until the contract is signed, it’s too early to discuss possible technologies that could be used. But, he added, without providing further details, the company has “several alternatives to a cutoff wall.”

Photo credit: SAFIN HAMED/AFP/Getty

Map credit: Adapted from Al-Ansari et al. study on possibility of Mosul Dam failure (“Mystery of Mosul Dam the Most Dangerous Dam in the World: Dam Failure and its Consequences”).

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Mapped: The Crumbling Iraqi Dam That May Flood The Country

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